Titanium oxide particles and titanium oxide type composite oxide particles are applied to various uses taking advantage of their chemical properties.
For example, these particles are used for oxidation-reduction catalysts or carriers, decorative materials or plastic surface coating agents utilizing ultraviolet screening power, anti-reflection coating materials utilizing high refractive index, antistatic materials utilizing electrical conductivity, functional hard coating materials utilizing a combination of these effects, and antibacterial agents, antifouling agents or super hydrophilic coating films utilizing photocatalytic actions. In recent years, further, titanium oxide has been favorably used for photocatlaysts or so-called photoelectric conversion materials for converting light energy into electric energy because it has high band gap. Moreover, titanium oxide has been used also for secondary batteries such as lithium batteries, hydrogen occlusion materials, proton conductive materials, etc.
For the titanium oxide and the titanium oxide type composite oxides applied to various uses as described above, a great number of functions are required. For example, when the titanium oxide is used as a catalyst, not only activity to a main reaction but also selectivity, mechanical strength, heat resistance, acid resistance and durability are required. When the titanium oxide is used as a decorative material, not only ultraviolet screening effect but also smoothness, touch, transparency, etc. are required.
When the titanium oxide is used as a coating material, more improved film forming properties, adhesion properties, film hardness, mechanical strength, abrasion resistance, etc. are required in addition to transparency and high refractive index.
From the above viewpoints, nano-tubular crystalline titanium oxide has received attention, and for example, nano-tubular crystalline titania having high specific surface area has been proposed in Japanese Patent Laid-Open Publication No. 152323/1998.
In this publication, it is disclosed that a crystalline titania powder is contacted with an alkali and then if necessary subjected to heat (calcining) treatment to prepare nano-tubular crystalline titania. However, even if the process described in this publication, e.g., a process described in the working example, is faithfully carried out, spherical particles or agglomerated particles are produced in addition to tubular particles and they are contained in the resulting crystalline titania particles, so that the yield of nano-tubular crystalline titania is low. Moreover, because the amount of residual sodium is large, sufficient performance of a catalyst, a catalyst carrier, a photocatalyst or the like cannot be obtained, and in some cases, any performance is not exhibited at all. Thus, there are many problems in this process.
Under such circumstances, the present inventors have earnestly studied a process for preparing tubular crystalline titanium oxide particles, and as a result, they have found that in Japanese Patent Laid-Open Publication No. 152323/1998, a powder of crystalline titanium oxide, particularly a powder (crystalline titania powder) obtained by calcining a powder prepared by a sol-gel process at a high temperature, is used as a starting material, but by the use of the powder as a starting material, desired tubular crystalline titanium oxide particles are not obtained. The present inventors have further studied, and as a result, they have found that hydrothermal treatment of a titanium oxide sol, in which particles of specific particle diameters are dispersed, in the presence of an alkali makes it possible to obtain tubular titanium oxide particles in an extremely high yield without producing agglomerates or spherical particles.
Photoelectric Conversion Materials
It is also known that titanium oxide is used for semiconductors for photoelectric conversion materials of solar cells. Ordinary solar cells are constituted in the following manner. First, a semiconductor film for a photoelectric conversion material, on which a photosensitizer has been adsorbed, is formed as an electrode on a support such as a glass plate having been coated with a transparent conductive film, then another support such as a glass plate having been coated with a transparent conductive film as a counterpart electrode is arranged, and an electrolyte is enclosed between these electrodes.
When the photosensitizer adsorbed on the semiconductor for a photoelectric conversion material is irradiated with sunlight, the photosensitizer absorbs light of visible region to excite electrons in the photosensitizer. The thus excited electrons move to the semiconductor, then pass through the transparent conductive glass electrode and move to the counterpart electrode. The electrons having moved to the counterpart electrode reduce the oxidation-reduction system (specifically, solvent, ionic compound, etc. contained in the electrolyte) in the electrolyte. On the other hand, the photosensitizer from which the electrons have moved to the semiconductor is in a state of oxidant, and this oxidant is reduced by the oxidation-reduction system in the electrolyte and thereby returns to the original state. The electrons continuously flow in this manner, whereby the solar cell using the semiconductor for a photoelectric conversion material is driven.
As the photoelectric conversion material, a material wherein a photosensitizing dye having absorption in the visible light region is adsorbed on a semiconductor surface is employed. For example, a solar cell having a layer of a color developing agent such as a transition metal complex on a surface of a metal oxide semiconductor is described in Japanese Patent Laid-Open-Publication No. 220380/1989. In National Publication of International Patent No. 504023/1993, a solar cell having a layer of a photosensitizing dye such as a transition metal complex on a surface of a titanium oxide semiconductor layer doped with metallic ion is disclosed.
In order to enhance photoelectric conversion efficiency, it is important for the solar cells mentioned above that moving of electrons from the light-absorbed and excited photosensitizing dye layer to the titania film is rapidly carried out. If the moving of electrons is not carried out rapidly, recombination of a ruthenium complex with the electrons takes place to lower the photoelectric conversion efficiency.
In order to solve such a problem, the present applicant has proposed novel photovoltaic cells in Japanese Patent Laid-Open Publication No. 339867/1999, Japanese Patent Laid-Open Publication No. 77691/2000, Japanese Patent Laid-Open Publication No. 155791/2001 and Japanese Patent Application No. 123065/2001. However, photovoltaic cells having higher photoelectric conversion efficiency are desired.
Photocatalysts
Recently, articles utilizing photocatalytic action of titania have received attention. For example, tiles having titania films formed on their surfaces, curtains containing titania and deodorants wherein titania is supported on activated carbon or zeolite are on the market and have been popular.
These articles are all aiming at antifouling, antibacterial or deodorizing effect by decomposing contaminants, bacteria or odorous matters adhering onto their surfaces utilizing the photocatalytic action of titania.
It is said that when titania particles are irradiated with ultraviolet rays, electrons or holes are produced inside the particle and they are diffused onto the particle surface and function as an oxidizing agent or a reducing agent, that is, the photocatalytic action of titania is due to the oxidative effect or the reducing effect.
The titania coating film having such photocatalytic action needs to have a large thickness in order to increase photocatalytic activity. Further, in order that the electrons or the holes produced inside the particle by the light irradiation move rapidly to the surface of the coating film, the coating film needs to have denseness. Therefore, high-temperature treatment is carried out in the film formation process to promote fusion bonding of particles, whereby denseness of the resulting coating film is increased and hardness thereof is also increased. However, if the treating temperature is raised, the crystal structure of titania changes from anatase type to rutile type, and there is brought about a problem that the photocatalytic activity is lowered. Moreover, there is another problem that it is difficult to form the titania coating film having the photocatalytic action on a material having no heat resistance, such as glass, plastic, wood, fiber or cloth, because the coating film is treated at a high temperature in the film forming process.
On this account, an attempt to use titania particles having been subjected to high-temperature treatment in advance was made. The titania particles, however, have a disadvantage that when they are subjected to high-temperature treatment, they come to have large diameters, high refractive index and wide light scattering, and therefore, a film of high transparency cannot be obtained.
Under such circumstances, the present inventors have earnestly studied, and as a result, they have found that the above problems of the photovoltaic cells and the photocatalysts can be all solved by the use of tubular titanium oxide particles of the present invention. Based on the finding, the present invention has been accomplished.
It is an object of the present invention to provide a process for preparing tubular titanium oxide particles useful as catalysts, catalyst carriers, adsorbents, photocatalysts, decorative materials, optical materials, photoelectric conversion materials, etc., and tubular titanium oxide particles. It is another object of the present invention to provide photovoltaic cells and photocatalysts using the tubular titanium oxide particles.